This invention relates to a method of providing coatings on component parts of an evaporator for use in air-conditioning systems for automotive vehicles.
Conventionally, an evaporator of this kind has been proposed in Japanese Provisional Patent Publication (Kokai) No. 57-162605. This evaporator is a laminate type comprising a plurality of tube elements, each of which is composed of a pair of stamped plates joined together in a face-to-face manner to define refrigerant-evaporating passages therebetween, and a plurality of corrugated fins interposed between adjacent tube elements to divide a space between the adjacent tube elements into a plurality of air passages, the tube elements and the corruated fins being superposed one upon another in an alternate manner and united together by brazing. The outer surfaces of the tube elements and the corrugated fins are coated with hydrophilic coating layers. The hydrophilic coating layers are formed by immersing the tube elements and the corrugated fins in a bath of an aqueous solution containing water glass and macromolecular silica, and then heating them to dry.
The laminate type evaporator is intended to have improved hydrophilic property of the surfaces of the corrugated fins and tube elements by coating same with the hydrophilic coating layers so as to cause condensate to be deposited on the surfaces in a flat form to thereby reduce the resistance of the condensate itself to the air flow and improve the adherability and mobility of the condensate on the surfaces, while restraining formation of bridges of the condensate between bent portions of the corrugated fins.
If the concentration of water glass and colloidal silica in the bath is made higher, the water glass (alkali silicate) K.sub.2 O.multidot.3SiO.sub.2 and colloidal silica SiO.sub.2 deposited on the surfaces of the tube elements and corrugated fins have increased weight per unit area after drying by heating. The hydrophilic property of the outer surfaces of the tube elements and corrugated fins is enhanced with increase in the amount of silicon dioxide SiO.sub.2 contained in the solid deposition to contribute to prevention of scattering of the condensate. More specifically, it is generally recognized that the smaller a contact angle .theta. formed between the surface of a solid object and the surface of a liquid drop on the solid object (an angle formed at a point where the surface of a liquid drop is in contact with the surface of a solid object between a tangent to the liquid drop surface at the point and the solid object surface), the more excellent the hydrophilic property of the surface of the solid object. As the SiO.sub.2 content in the solid deposition is higher, the angle .theta. becomes smaller as shown in FIG. 6A, and accordingly the hydrophilic property of the outer surfaces of the tube elements and corrugated fins becomes more excellent, whereas as the SiO.sub.2 content in the solid deposition is lower, the angle .theta. becomes larger as shown in FIG. 6B, and accordingly the hydrophilic property becomes poorer.
However, if the SiO.sub.2 content is too high, not only moisture but also an offensive smell is attached to the outer surfaces of the corrugated fins and tube elements during operation of the air-conditioning system of the automotive vehicle. When the air-conditioning system is stopped, there occurs exhalation of not only the moisture but also the offensive smell attached to the surfaces. As a result, the offensive smell is fed into the car compartment. Therefore, it is necessary to limit the content of water glass and colloidal silica in the hydrophilic coating layers formed on the outer surfaces of the tube elements and corrugated fins of the evaporator within such a range as to contribute to prevention of scattering of the condensate and at the same time prevention of occurrence of the offensive smell.